Tandem RCM-Pauson-Khand reaction for access to tricycles in one step

Rhodium-Catalyzed Enantioselective and Desymmetrizative Pauson-Khand Reaction: Access to Tricyclo[6.2.1.04,11]undecenes

Rhodium-catalyzed asymmetric desymmetrization Pauson-Khand reaction of C4-alkynyl-tethered cyclohexadienones has been developed as a novel strategy for access to fused 6-5-5 tricycles bearing three consecutive stereogenic centers. An array of chiral tricyclo[6.2.1.0^4,11]undecenes have been synthesized in high yields and enantioselectivities in a single step under mild conditions. This strategy employs readily accessible internal-olefin-containing 1,6-enynes, providing a potentially powerful tool for constructing chiral polycyclic scaffolds of complex molecules containing cyclopentenones and cyclohexenones.

The Domino Way to Heterocycles

Sequential transformations enable the facile synthesis of complex target molecules from simple building blocks in a single preparative step. Their value is amplified if they also create multiple stereogenic centers. In the ongoing search for new domino processes, emphasis is usually placed on sequential reactions which occur cleanly and without forming by-products. As a prerequisite for an ideally proceeding one-pot sequential transformation, the reactivity pattern of all participating components has to be such that each building block gets involved in a reaction only when it is supposed to do so. The development of sequences that combine transformations of fundamentally different mechanisms broadens the scope of such procedures in synthetic chemistry. This mini review contains a representative sampling from the last 15 years on the kinds of reactions that have been sequenced into cascades to produce heterocyclic molecules.

Step-wise and pre-organization induced synthesis of a crossed alkene-bridged nisin Z DE-ring mimic by ring-closing metathesis

This paper describes two approaches for the synthesis of a crossed alkene-bridged mimic of the thioether ring system of the nisin Z DE-fragment. The first approach comprised the stepwise total synthesis featuring a cross metathesis and a macrolactamization on a solid support followed by a ring-closing metathesis in solution. Via this route the title compound was obtained in an overall yield of 7% (85% on average for 16 reaction steps). In the second approach, the linear precursor peptide was subjected to ring-closing metathesis and the bicyclic peptide with the correct side chain connectivity pattern was obtained in yields up to 95%. The preferred formation of the bicyclic crossed alkene-bridged mimic of the DE-ring suggests a favorable pre-organization of the linear precursor peptide.

The Pauson-Khand reaction, a powerful synthetic tool for the synthesis of complex molecules

There are still some synthetic chemists who hesitate to use metal-mediated or -catalysed reactions. The Pauson-Khand reaction (PKR) is a powerful transformation that has now been sufficiently well developed to be routinely considered when planning a synthesis, especially of polycyclic complex molecules. This tutorial review aims to encourage the use of this process explaining the best ways of performing a PKR both in the stoichiometric and the catalytic version, showing the scope of the process and its limitations. Additionally, asymmetry can be introduced in the reaction using several strategies, which will be discussed. The most recent examples of the synthetic applications of the PKR in natural product synthesis will give the reader an idea of the great usefulness of this reaction.